Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Method for Activating or Reactivating
Sil~er-Carrier-Ca-talysts _ _
Silver catalysts are used to produce ethylene oxi~e by oxidizing
ethylene with oxygen or gases containing oxygen. It i9 also kn~n to mix
such catalysts with so-called promotors, more partic~arly alkaline~earth
metals, prefera.bly barium compounds and/or alkaline metals, especially so-
called heavy alkaline metals such as rubldium and/or cesl~ (German
OS 23 00 512~ 27 33 688~ 24 42 568). These promotors are generally applied
simultaneously with the silver, or after the silver has been applied to the
carrier, during production o~ the catalyst.
It is also kno~n that, in course of time, silver catalysts lose `~
their selectivity, ~nd that, after a ~ew years, they must be replaced with
new catalysts. Replacing a catalyst, the performance of which has dimin-
ished, with a new catalyst in large scale installations is both time-consum-
ing and labour intensivej it also halts production and is costly.
- It is also known to improve the performance of a silver catalyst,
or to reactivate a used catalyst (German OS 27 46 976, German AS 25 19 599).
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~ This may be achie~ed in large-scale installations by ~looding the reactor
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containing the catalyst with a solution of the promotor-metal compound.
After the excess solvent has been removed, residual sol~ent is eliminated by
heating and blowing an inert gas through the catalyst bed. This method re-
quires the use of solvents and also halts production.
It is the purpo:e of the invention to provide a simple method of
improving the performance of catalysts, maintaining their performance as long
as possible, and avoiding replacement with a ne~ catalyst, or postponing such
replacement as long as possible.
-~ This purpose is achieved in that the reaction-gas mixture is
passed, at least intermi-ttently and at least partly, through a layer arranged
before the catalyst zone~ the said layer containing compounds of the pro~-
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otor-metals in a uniformly divided orm.
Accordingly, the present invention provides a method for activating
or reactivating a supported silver cakalyst for the production of ethylene oxide
by oxidizing ethylene with a reaction gas comprising oxygen or an oxygen- ;
containing gas, with the aid of a promotor metal, characterized in that the
mi~ture or ethylene and reaction gas is passed at least intermittently and at
least partly through a layer arranged before the catalyst zone, said layer
containing a promotor metal compound in uniformly divided form or a carrier
mate*ial impregnated with the promotor metal compound.
It is desirable to use promotor metal compounds w~ich are volatile
under reaction conditions.
The method is applicable both to silver catalysts already containing
promotors, for the purpose of maintaining their performance, and to such
catalysts which do not contain promotors, for the purpose of improving their
performance.
The promotor metal compounds are used in coarsely granular form.
In particular, use is made of a carrier layer, corresponding to the catalyst-
carrier, which has been impregnated with a solution of a promotor metal compound.
This layer may be arranged directly in the individual reaction -tubes
at the beginning of the catalyst zone, i.e. at the reaction gas inlet. It is
also psssible to design the layer so that it may be easily replaced. In one
particular configuration, the said layer is arranged in a separate container
located in the path of ~he reaction gas, as close as possible to, and directly
in front of, the catalyst zone. According to another configuration, the
carrier may also be located in ~he part flow of reaction gas, the advantage
of this being that the part fIow need be operated only intermittently. It is
desirable for the layer in the reaction tube or separate con-
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tainer to comprise first a layer of pure carrier-material, then a layer of
promotor-metal compounds, followed by a carrier-layer impregnated with pro-
motor-metal co~lpounds, Pnd finally another carrler-layer.
The adrantage of this is that the temperature of the reaction-gas
may be varied during its passage through the reactor. Both the container and
the reaction-tube are, or may be~ equipped with appropriate-heating and con-
trol-units.
The a~lount of individual promo-tor-metal compounds may be ad~usted
to the desired proportion. ~he thickness of the layer impregnated with pro-
motor~metal compounds, in the reaction tubes, is generally between 1 and 20,more particularly between 5 and 10%, of the length of the tube.
The followin~, in particular, are suitable promotor-metal com-
~ pounds: barium compounds such as barium-oxide, -peroxide 9 -hydroxide, -nit-
;~ rite, -nitrate, -carbonate, -acetate~ -oxalate, -tartrate, -naphthenate~ ~;
-stearate, -dodecanate, -chloride and/or alkaline-metal compounds of rubidium
andlor cesiu~ such as o~ides, peroxides, hydroxides, nitrites, nitrates,
- carbonates, acetates, oxalates~ tartra~es, stearates, dodecanates and chlo-
rides.
~he following examples e~plain the method according to the inven-
2~ tion.
- The experimental reactor consists of a reaction-tube 6000 mm in
:- length and 26 mm 1n diameter made o~ stainless steel. One tube was filled
with the catalyst described in greater detail in the folIowing examples. A
second tube was filled with the same catalyst and a layer was also arranged
at the gas-inlet, the said layer consisting, in the ~irection of the flow of
gas, of 400 mm of untreated carrier-material, 450 mm of a carrier-material
coated with promotor-me-talsland 30 mm of untreated carrier-material. The
tubes ~Tere enclosed in a jacket containing water or steam to carry away the
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heat o~ the reaction. The catalysts each contained 19.5$ by weight of sil-
ver. The carrier-material used was an aluminum-oxide containing a~ou-t 12% by
weight o~ silicon-dioxide. The reaction tubes were operated ~imultaneously
with the same mixture of gas, of the following composition:
25% by volume of C
49% by volu~e of CHL~
6.8% by volume of 2
4.5~ by volume of C02
0.?% by volume of C2H6
Remainder Ar ~ N7
Ex~ple 1
Use was made o~ 2.4 kg of the silver catalyst described above and
containing no additional promotors. ~he impregnated carrier-layer in the
second reaction-tube contained 200 g Or carrier-material and 1.1 g of potas-
siuml 1.6 g of baril~, and 0.~4 g of cesium~ applied in the form of an aque~
ous solution, rendered alkaline with potassium-carbona~e, o~ barium-peroxide
and cesium-nitrate.
In both cases, the gas-lo&d contained, at the end of the inlet-
phase, 9.26 ~m3/1 of catalyst at a positive pressure of 19.8 bars~ corre-
sponding to an overall gas-load of 25 ~m3/h. The chlorine content in the
circulating gas was held to between 7 and 8 mg of Cl/Nm3, with the aid of
1,2-dichlorethane. The ~team!chamber temperature was adjusted to produce an
ethylene-oxide concentration averagine 1.6% by volume at the reactor outlet.
Start A~ter 6 weeks
of operation
_ Selectivity ~emp. Selectivity Temp.
Mol-% C Mol-% C
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Comparison 74.0 25471.6 271
Invention 74.3 25274.8 251
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Exam~le 2
The comparison catalyst used already contained 472 ppm of barium.
Under othe~wise similar conditionsg the following results were obtained:
Start After 6 weeks
of operation
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; Selectivity Temp. Selecti~ity Temp.
Mol-% C Mol-% C
, _ . .. ... . . __ . _
Comparison 75-7 249 73-5 267
Invention 75-8 249 76.7 247 ~ -
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The comparison catalyst used alread~ contained 84l ppm o~ barium
- and 91 ppm of cesium. Under othe~ise similar conditions, the following re-
sults were obtained:
Start ~ After 6 weeks
of operation
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Selectivity Temp. Selectivity ~emp.
' Mol-% C Mol~% C
.. _ _. _
Gomparison 80.4 245 77-5 265
Inve~tion 80.3 245 80.3 245
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Example 4
The co~parison catalyst used was a pure silver catalyst as in Ex-
ample l. According to the i~vention, the carrier-layer contained 0.29 g of
rubidium instead of ceslum. The following results were obtained:
Start After 6 weeks
of operation
_ ., ._____ _ .
Selecti~ity Temp. Selectivity Temp.
Mol-% C Mol-% C
Comparison 74.0 254 71.6 271
Invention 74.2 252 73.7 255
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